The present invention relates to a thread cutter assembly for a sewing machine. More particularly, the invention relates to an improved mechanism for mounting and driving a movable edge which is adapted to cut predetermined needle and bobbin threads in cooperation with a stationary edge.
Cutting off the needle thread and bobbin thread from pieces of cloth on which stitching has been completed is one of the important steps in a sewing operation. However, it is desirable to improve the cutting operation because it takes a relatively long time. Accordingly, a recent high-speed sewing machine has been provided with an automatic thread cutting mechanism. With this mechanism, the needle and bobbin threads are quickly and accurately cut off the pieces of sewn cloth in response to a thread cutting instruction which is issued, for instance, when the operator steps on a pedal. This mechanism contributes greatly to the high-speed sewing operation.
In a conventional automatic thread cutting type sewing machine, the relative movement of the stationary edge and the movable edge for cutting the threads is effected directly by the oscillating shaft of the sewing machine. Accordingly, the conventional sewing machine suffers from problems in that a large driving-power transmitting mechanism for driving the movable edge is required and hence the sewing machine is necessarily bulky and often is subject to breakdown.
In a two-needle sewing machine for carrying out sewing operations with two needles simultaneously, two thread cutting mechanisms provided respectively for the two needles are driven directly by the common oscillating shaft. Therefore, the characteristics and especially the thread cutting timing of the two thread cutting mechanisms cannot be adjusted separately. Thus, the two-needle sewing machine is disadvantageous in that a single thread cutting mechanism cannot satisfactorily cut the threads. In addition, since the thread cutting operations for the two needles are carried out by the common thread cutting drive mechanism, when the stitch pattern or length is adjusted, it is necessary to adjust the thread cutting mechanism also.
In the conventional thread cutter assembly of the sewing machine of this type, the thread cutting edges are either provided in a vertical plane so that they are parallel to the side of the bobbin carrying case where a bobbin thread pull-out hole is formed or they are provided in a horizontal plane perpendicular to the side of the bobbin carrying case. However, in the two-needle sewing machine capable of simultaneously carrying out sewing operations with two needles, rotary hook assemblies are so provided as to rotate in a horizontal plane and the bobbin thread extends towards the throat plate through the bobbin thread pull-out hole of the bobbin carrying case. Therefore, if the thread cutting edges are provided in a horizontal plane by the side of the bobbin carrying case, the angle formed between the bobbin thread and the thread cutting edges is necessarily small which tends to cause a problem that the thread cannot be cut with the thread cutting edges.